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| author | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-05-22 05:12:18 +0000 |
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| committer | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-05-22 05:12:18 +0000 |
| commit | aa0f5b38aec14428b4b80e06f90ff781f8bca5f1 (patch) | |
| tree | 95f3d0c8cb0d59d88900dbbd72110d7ab6e15b2a /doc/src/ipsec.html | |
| parent | 7c383bc22113b23718be89fe18eeb251942d7356 (diff) | |
| download | vyos-strongswan-aa0f5b38aec14428b4b80e06f90ff781f8bca5f1.tar.gz vyos-strongswan-aa0f5b38aec14428b4b80e06f90ff781f8bca5f1.zip | |
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diff --git a/doc/src/ipsec.html b/doc/src/ipsec.html new file mode 100644 index 000000000..4647eaf66 --- /dev/null +++ b/doc/src/ipsec.html @@ -0,0 +1,1206 @@ +<html> +<head> + <meta http-equiv="Content-Type" content="text/html"> + <title>IPsec protocols</title> + <meta name="keywords" + content="Linux, IPsec, VPN, security, FreeSWAN, protocol, ESP, AH, IKE"> + <!-- + + Written by Sandy Harris for the Linux FreeS/WAN project + Freely distributable under the GNU General Public License + + More information at www.freeswan.org + Feedback to users@lists.freeswan.org + + CVS information: + RCS ID: $Id: ipsec.html,v 1.1 2004/03/15 20:35:24 as Exp $ + Last changed: $Date: 2004/03/15 20:35:24 $ + Revision number: $Revision: 1.1 $ + + CVS revision numbers do not correspond to FreeS/WAN release numbers. + --> +</head> + +<body> +<h1><a name="ipsec.detail">The IPsec protocols</a></h1> + +<p>This section provides information on the IPsec protocols which FreeS/WAN +implements. For more detail, see the <a href="rfc.html">RFCs</a>.</p> + +<p>The basic idea of IPsec is to provide security functions, <a +href="glossary.html#authentication">authentication</a> and <a +href="glossary.html#encryption">encryption</a>, at the IP (Internet Protocol) +level. This requires a higher-level protocol (IKE) to set things up for the +IP-level services (ESP and AH).</p> + +<h2>Protocols and phases</h2> + +<p>Three protocols are used in an IPsec implementation:</p> +<dl> + <dt>ESP, Encapsulating Security Payload</dt> + <dd>Encrypts and/or authenticates data</dd> + <dt>AH, Authentication Header</dt> + <dd>Provides a packet authentication service</dd> + <dt>IKE, Internet Key Exchange</dt> + <dd>Negotiates connection parameters, including keys, for the other + two</dd> +</dl> + +<p>The term "IPsec" (also written as IPSEC) is slightly ambiguous. In some +contexts, it includes all three of the above but in other contexts it refers +only to AH and ESP.</p> + +<p>There is more detail below, but a quick summary of how the whole thing +works is:</p> +<dl> + <dt>Phase one IKE (main mode exchange)</dt> + <dd>sets up a keying channel (ISAKMP SA) between the two gateways</dd> + <dt>Phase two IKE (quick mode exchange)</dt> + <dd>sets up data channels (IPsec SAs)</dd> + <dt>IPsec proper</dt> + <dd>exchanges data using AH or ESP</dd> +</dl> + +<p>Both phases of IKE are repeated periodically to automate re-keying.</p> + +<h2><a name="others">Applying IPsec</a></h2> + +<p>Authentication and encryption functions for network data can, of course, +be provided at other levels. Many security protocols work at levels above +IP.</p> +<ul> + <li><a href="glossary.html#PGP">PGP</a> encrypts and authenticates mail + messages</li> + <li><a href="glossary.html#SSH">SSH</a> authenticates remote logins and + then encrypts the session</li> + <li><a href="glossary.html#SSL">SSL</a> or <a + href="glossary.html#TLS">TLS</a> provides security at the sockets layer, + e.g. for secure web browsing</li> +</ul> + +<p>and so on. Other techniques work at levels below IP. For example, data on +a communications circuit or an entire network can be encrypted by specialised +hardware. This is common practice in high-security applications.</p> + +<h3><a name="advantages">Advantages of IPsec</a></h3> + +<p>There are, however, advantages to doing it at the IP level instead of, or +as well as, at other levels.</p> + +<p>IPsec is the <strong>most general way to provide these services for the +Internet</strong>.</p> +<ul> + <li>Higher-level services protect a <em>single protocol</em>; for example + PGP protects mail.</li> + <li>Lower level services protect a <em>single medium</em>; for example a + pair of encryption boxes on the ends of a line make wiretaps on that line + useless unless the attacker is capable of breaking the encryption.</li> +</ul> + +<p>IPsec, however, can protect <em>any protocol</em> running above IP and +<em>any medium</em> which IP runs over. More to the point, it can protect a +mixture of application protocols running over a complex combination of media. +This is the normal situation for Internet communication; IPsec is the only +general solution.</p> + +<p>IPsec can also provide some security services "in the background", with +<strong>no visible impact on users</strong>. To use <a +href="glossary.html#PGP">PGP</a> encryption and signatures on mail, for +example, the user must at least:</p> +<ul> + <li>remember his or her passphrase,</li> + <li>keep it secure</li> + <li>follow procedures to validate correspondents' keys</li> +</ul> + +<p>These systems can be designed so that the burden on users is not onerous, +but any system will place some requirements on users. No such system can hope +to be secure if users are sloppy about meeting those requirements. The author +has seen username and password stuck on terminals with post-it notes in an +allegedly secure environment, for example.</p> + +<h3><a name="limitations">Limitations of IPsec</a></h3> + +<p>IPsec is designed to secure IP links between machines. It does that well, +but it is important to remember that there are many things it does not do. +Some of the important limitations are:</p> +<dl> + <dt><a name="depends">IPsec cannot be secure if your system isn't</a></dt> + <dd>System security on IPsec gateway machines is an essential requirement + if IPsec is to function as designed. No system can be trusted if the + underlying machine has been subverted. See books on Unix security such + as <a href="biblio.html#practical">Garfinkel and Spafford</a> or our + web references for <a href="web.html#linsec">Linux security</a> or more + general <a href="web.html#compsec">computer security</a>. + <p>Of course, there is another side to this. IPsec can be a powerful + tool for improving system and network security. For example, requiring + packet authentication makes various spoofing attacks harder and IPsec + tunnels can be extremely useful for secure remote administration of + various things.</p> + </dd> + <dt><a name="not-end-to-end">IPsec is not end-to-end</a></dt> + <dd>IPsec cannot provide the same end-to-end security as systems working + at higher levels. IPsec encrypts an IP connection between two machines, + which is quite a different thing than encrypting messages between users + or between applications. + <p>For example, if you need mail encrypted from the sender's desktop to + the recipient's desktop and decryptable only by the recipient, use <a + href="glossary.html#PGP">PGP</a> or another such system. IPsec can + encrypt any or all of the links involved -- between the two mail + servers, or between either server and its clients. It could even be + used to secure a direct IP link from the sender's desktop machine to + the recipient's, cutting out any sort of network snoop. What it cannot + ensure is end-to-end user-to-user security. If only IPsec is used to + secure mail, then anyone with appropriate privileges on any machine + where that mail is stored (at either end or on any store-and-forward + servers in the path) can read it.</p> + <p>In another common setup, IPsec encrypts packets at a security + gateway machine as they leave the sender's site and decrypts them on + arrival at the gateway to the recipient's site. This does provide a + useful security service -- only encrypted data is passed over the + Internet -- but it does not even come close to providing an end-to-end + service. In particular, anyone with appropriate privileges on either + site's LAN can intercept the message in unencrypted form.</p> + </dd> + <dt><a name="notpanacea">IPsec cannot do everything</a></dt> + <dd>IPsec also cannot provide all the functions of systems working at + higher levels of the protocol stack. If you need a document + electronically signed by a particular person, then you need his or her + <a href="glossary.html#signature">digital signature</a> and a <a + href="glossary.html#public">public key cryptosystem</a> to verify it + with. + <p>Note, however, that IPsec authentication of the underlying + communication can make various attacks on higher-level protocols more + difficult. In particular, authentication prevents <a + href="glossary.html#middle">man-in-the-middle attacks</a>.</p> + </dd> + <dt><a name="no_user">IPsec authenticates machines, not users</a></dt> + <dd>IPsec uses strong authentication mechanisms to control which messages + go to which machines, but it does not have the concept of user ID, + which is vital to many other security mechansims and policies. This + means some care must be taken in fitting the various security + mechansims on a network together. For example, if you need to control + which users access your database server, you need some non-IPsec + mechansim for that. IPsec can control which machines connect to the + server, and can ensure that data transfer to those machines is done + securely, but that is all. Either the machines themselves must control + user access or there must be some form of user authentication to the + database, independent of IPsec.</dd> + <dt><a name="DoS">IPsec does not stop denial of service attacks</a></dt> + <dd><a href="glossary.html#DOS">Denial of service</a> attacks aim at + causing a system to crash, overload, or become confused so that + legitimate users cannot get whatever services the system is supposed to + provide. These are quite different from attacks in which the attacker + seeks either to use the service himself or to subvert the service into + delivering incorrect results. + <p>IPsec shifts the ground for DoS attacks; the attacks possible + against systems using IPsec are different than those that might be used + against other systems. It does not, however, eliminate the possibility + of such attacks.</p> + </dd> + <dt><a name="traffic">IPsec does not stop traffic analysis</a></dt> + <dd><a href="glossary.html#traffic">Traffic analysis</a> is the attempt + to derive intelligence from messages without regard for their contents. + In the case of IPsec, it would mean analysis based on things visible in + the unencrypted headers of encrypted packets -- source and destination + gateway addresses, packet size, et cetera. Given the resources to + acquire such data and some skill in analysing it (both of which any + national intelligence agency should have), this can be a very powerful + technique. + <p>IPsec is not designed to defend against this. Partial defenses are + certainly possible, and some are <a href="#traffic.resist">described + below</a>, but it is not clear that any complete defense can be + provided.</p> + </dd> +</dl> + +<h3><a name="uses">IPsec is a general mechanism for securing IP</a></h3> + +<p>While IPsec does not provide all functions of a mail encryption package, +it can encrypt your mail. In particular, it can ensure that all mail passing +between a pair or a group of sites is encrypted. An attacker looking only at +external traffic, without access to anything on or behind the IPsec gateway, +cannot read your mail. He or she is stymied by IPsec just as he or she would +be by <a href="glossary.html#PGP">PGP</a>.</p> + +<p>The advantage is that IPsec can provide the same protection for <strong> +anything transmitted over IP</strong>. In a corporate network example, PGP +lets the branch offices exchange secure mail with head office. SSL and SSH +allow them to securely view web pages, connect as terminals to machines, and +so on. IPsec can support all those applications, plus database queries, file +sharing (NFS or Windows), other protocols encapsulated in IP (Netware, +Appletalk, ...), phone-over-IP, video-over-IP, ... anything-over-IP. The only +limitation is that IP Multicast is not yet supported, though there are +Internet Draft documents for that.</p> + +<p>IPsec creates <strong>secure tunnels through untrusted networks</strong>. +Sites connected by these tunnels form VPNs, <a +href="glossary.html#VPN">Virtual Private Networks</a>.</p> + +<p>IPsec gateways can be installed wherever they are required.</p> +<ul> + <li>One organisation might choose to install IPsec only on firewalls + between their LANs and the Internet. This would allow them to create a + VPN linking several offices. It would provide protection against anyone + outside their sites.</li> + <li>Another might install IPsec on departmental servers so everything on + the corporate backbone net was encrypted. This would protect messages on + that net from everyone except the sending and receiving department.</li> + <li>Another might be less concerned with information secrecy and more with + controlling access to certain resources. They might use IPsec packet + authentication as part of an access control mechanism, with or without + also using the IPsec encryption service.</li> + <li>It is even possible (assuming adequate processing power and an IPsec + implementation in each node) to make every machine its own IPsec gateway + so that everything on a LAN is encrypted. This protects information from + everyone outside the sending and receiving machine.</li> + <li>These techniques can be combined in various ways. One might, for + example, require authentication everywhere on a network while using + encryption only for a few links.</li> +</ul> + +<p>Which of these, or of the many other possible variants, to use is up to +you. <strong>IPsec provides mechanisms; you provide the policy</strong>.</p> + +<p><strong>No end user action is required</strong> for IPsec security to be +used; they don't even have to know about it. The site administrators, of +course, do have to know about it and to put some effort into making it work. +Poor administration can compromise IPsec as badly as the post-it notes +mentioned above. It seems reasonable, though, for organisations to hope their +system administrators are generally both more security-conscious than end +users and more able to follow computer security procedures. If not, at least +there are fewer of them to educate or replace.</p> + +<p>IPsec can be, and often should be, used with along with security protocols +at other levels. If two sites communicate with each other via the Internet, +then IPsec is the obvious way to protect that communication. If two others +have a direct link between them, either link encryption or IPsec would make +sense. Choose one or use both. Whatever you use at and below the IP level, +use other things as required above that level. Whatever you use above the IP +level, consider what can be done with IPsec to make attacks on the higher +levels harder. For example, <a href="glossary.html#middle">man-in-the-middle +attacks</a> on various protocols become difficult if authentication at packet +level is in use on the potential victims' communication channel.</p> + +<h3><a name="authonly">Using authentication without encryption</a></h3> + +<p>Where appropriate, IPsec can provide authentication without encryption. +One might do this, for example:</p> +<ul> + <li>where the data is public but one wants to be sure of getting the right + data, for example on some web sites</li> + <li>where encryption is judged unnecessary, for example on some company or + department LANs</li> + <li>where strong encryption is provided at link level, below IP</li> + <li>where strong encryption is provided in other protocols, above IP<br> + Note that IPsec authentication may make some attacks on those protocols + harder.</li> +</ul> + +<p>Authentication has lower overheads than encryption.</p> + +<p>The protocols provide four ways to build such connections, using either an +AH-only connection or ESP using null encryption, and in either manually or +automatically keyed mode. FreeS/WAN supports only one of these, manually +keyed AH-only connections, and <strong>we do not recommend using +that</strong>. Our reasons are discussed under <a +href="#traffic.resist">Resisting traffic analysis</a> a few sections further +along.</p> + +<h3><a name="encnoauth">Encryption without authentication is +dangerous</a></h3> + +<p>Originally, the IPsec encryption protocol <a +href="glossary.html#ESP">ESP</a> didn't do integrity checking. It only did +encryption. Steve Bellovin found many ways to attack ESP used without +authentication. See his paper <a +href="http://www.research.att.com/~smb/papers/badesp.ps">Problem areas for +the IP Security Protocols</a>. To make a secure connection, you had to add an +<a href="glossary.html#AH">AH</a> Authentication Header as well as ESP. +Rather than incur the overhead of several layers (and rather than provide an +ESP layer that didn't actually protect the traffic), the IPsec working group +built integrity and replay checking directly into ESP.</p> + +<p>Today, typical usage is one of:</p> +<ul> + <li>ESP for encryption and authentication</li> + <li>AH for authentication alone</li> +</ul> + +<p>Other variants are allowed by the standard, but not much used:</p> +<dl> + <dt>ESP encryption without authentication</dt> + <dd><strong>Bellovin has demonstrated fatal flaws in this. Do not + use.</strong></dd> + <dt>ESP encryption with AH authentication</dt> + <dd>This has higher overheads than using the authentication in ESP, and + no obvious benefit in most cases. The exception might be a network + where AH authentication was widely or universally used. If you're going + to do AH to conform with network policy, why authenticate again in the + ESP layer?</dd> + <dt>Authenticate twice, with AH and with ESP</dt> + <dd>Why? Of course, some folk consider "belt and suspenders" the sensible + approach to security. If you're among them, you might use both + protocols here. You might also use both to satisfy different parts of a + security policy. For example, an organisation might require AH + authentication everywhere but two users within the organisation might + use ESP as well.</dd> + <dt>ESP authentication without encryption</dt> + <dd>The standard allows this, calling it "null encryption". FreeS/WAN + does not support it. We recommend that you use AH instead if + authentication is all you require. AH authenticates parts of the IP + header, which ESP-null does not do.</dd> +</dl> + +<p>Some of these variants cannot be used with FreeS/WAN because we do not +support ESP-null and do not support automatic keying of AH-only +connections.</p> + +<p>There are fairly frequent suggestions that AH be dropped entirely from the +IPsec specifications since ESP and null encryption can handle that situation. +It is not clear whether this will occur. My guess is that it is unlikely.</p> + +<h3><a name="multilayer">Multiple layers of IPsec processing are +possible</a></h3> + +<p>The above describes combinations possible on a single IPsec connection. In +a complex network you may have several layers of IPsec in play, with any of +the above combinations at each layer.</p> + +<p>For example, a connection from a desktop machine to a database server +might require AH authentication. Working with other host, network and +database security measures, AH might be just the thing for access control. +You might decide not to use ESP encryption on such packets, since it uses +resources and might complicate network debugging. Within the site where the +server is, then, only AH would be used on those packets.</p> + +<p>Users at another office, however, might have their whole connection (AH +headers and all) passing over an IPsec tunnel connecting their office to the +one with the database server. Such a tunnel should use ESP encryption and +authentication. You need authentication in this layer because without +authentication the encryption is vulnerable and the gateway cannot verify the +AH authentication. The AH is between client and database server; the gateways +aren't party to it.</p> + +<p>In this situation, some packets would get multiple layers of IPsec applied +to them, AH on an end-to-end client-to-server basis and ESP from one office's +security gateway to the other.</p> + +<h3><a name="traffic.resist">Resisting traffic analysis</a></h3> + +<p><a href="glossary.html#traffic">Traffic analysis</a> is the attempt to +derive useful intelligence from encrypted traffic without breaking the +encryption.</p> + +<p>Is your CEO exchanging email with a venture capital firm? With bankruptcy +trustees? With an executive recruiting agency? With the holder of some +important patents? If an eavesdropper learns about any of those, then he has +interesting intelligence on your company, whether or not he can read the +messages themselves.</p> + +<p>Even just knowing that there is network traffic between two sites may tell +an analyst something useful, especially when combined with whatever other +information he or she may have. For example, if you know Company A is having +cashflow problems and Company B is looking for aquisitions, then knowing that +packets are passing between the two is interesting. It is more interesting if +you can tell it is email, and perhaps yet more if you know the sender and +recipient.</p> + +<p>Except in the simplest cases, traffic analysis is hard to do well. It +requires both considerable resources and considerable analytic skill. +However, intelligence agencies of various nations have been doing it for +centuries and many of them are likely quite good at it by now. Various +commercial organisations, especially those working on "targeted marketing" +may also be quite good at analysing certain types of traffic.</p> + +<p>In general, defending against traffic analysis is also difficult. +Inventing a really good defense could get you a PhD and some interesting job +offers.</p> + +<p>IPsec is not designed to stop traffic analysis and we know of no plausible +method of extending it to do so. That said, there are ways to make traffic +analysis harder. This section describes them.</p> + +<h4><a name="extra">Using "unnecessary" encryption</a></h4> + +<p>One might choose to use encryption even where it appears unnecessary in +order to make analysis more difficult. Consider two offices which pass a +small volume of business data between them using IPsec and also transfer +large volumes of Usenet news. At first glance, it would seem silly to encrypt +the newsfeed, except possibly for any newsgroups that are internal to the +company. Why encrypt data that is all publicly available from many sites?</p> + +<p>However, if we encrypt a lot of news and send it down the same connection +as our business data, we make <a href="glossary.html#traffic">traffic +analysis</a> much harder. A snoop cannot now make inferences based on +patterns in the volume, direction, sizes, sender, destination, or timing of +our business messages. Those messages are hidden in a mass of news messages +encapsulated in the same way.</p> + +<p>If we're going to do this we need to ensure that keys change often enough +to remain secure even with high volumes and with the adversary able to get +plaintext of much of the data. We also need to look at other attacks this +might open up. For example, can the adversary use a chosen plaintext attack, +deliberately posting news articles which, when we receive and encrypt them, +will help break our encryption? Or can he block our business data +transmission by flooding us with silly news articles? Or ...</p> + +<p>Also, note that this does not provide complete protection against traffic +analysis. A clever adversary might still deduce useful intelligence from +statistical analysis (perhaps comparing the input newsfeed to encrypted +output, or comparing the streams we send to different branch offices), or by +looking for small packets which might indicate establishment of TCP +connections, or ...</p> + +<p>As a general rule, though, to improve resistance to traffic analysis, you +should <strong>encrypt as much traffic as possible, not just as much as seems +necessary.</strong></p> + +<h4><a name="multi-encrypt">Using multiple encryption</a></h4> + +<p>This also applies to using multiple layers of encryption. If you have an +IPsec tunnel between two branch offices, it might appear silly to send <a +href="glossary.html#PGP">PGP</a>-encrypted email through that tunnel. +However, if you suspect someone is snooping your traffic, then it does make +sense:</p> +<ul> + <li>it protects the mail headers; they cannot even see who is mailing + who</li> + <li>it protects against user bungles or software malfunctions that + accidentally send messages in the clear</li> + <li>it makes any attack on the mail encryption much harder; they have to + break IPsec or break into your network before they can start on the mail + encryption</li> +</ul> + +<p>Similar arguments apply for <a href="glossary.html#SSL">SSL</a>-encrypted +web traffic or <a href="glossary.html#SSH">SSH</a>-encrypted remote login +sessions, even for end-to-end IPsec tunnels between systems in the two +offices.</p> + +<h4><a name="fewer">Using fewer tunnels</a></h4> + +<p>It may also help to use fewer tunnels. For example, if all you actually +need encrypted is connections between:</p> +<ul> + <li>mail servers at branch and head offices</li> + <li>a few branch office users and the head office database server</li> +</ul> + +<p>You might build one tunnel per mail server and one per remote database +user, restricting traffic to those applications. This gives the traffic +analyst some information, however. He or she can distinguish the tunnels by +looking at information in the ESP header and, given that distinction and the +patterns of tunnel usage, might be able to figure out something useful. +Perhaps not, but why take the risk?</p> + +<p>We suggest instead that you build one tunnel per branch office, encrypting +everything passing from head office to branches. This has a number of +advantages:</p> +<ul> + <li>it is easier to build and administer</li> + <li>it resists traffic analysis somewhat better</li> + <li>it provides security for whatever you forgot. For example, if some user + at a remote office browses proprietary company data on some head office + web page (that the security people may not even know about!), then that + data is encrypted before it reaches the Internet.</li> +</ul> + +<p>Of course you might also want to add additional tunnels. For example, if +some of the database data is confidential and should not be exposed even +within the company, then you need protection from the user's desktop to the +database server. We suggest you do that in whatever way seems appropriate -- +IPsec, SSH or SSL might fit -- but, whatever you choose, pass it between +locations via a gateway-to-gateway IPsec tunnel to provide some resistance to +traffic analysis.</p> + +<h2><a name="primitives">Cryptographic components</a></h2> + +<p>IPsec combines a number of cryptographic techniques, all of them +well-known and well-analyzed. The overall design approach was conservative; +no new or poorly-understood components were included.</p> + +<p>This section gives a brief overview of each technique. It is intended only +as an introduction. There is more information, and links to related topics, +in our <a href="glossary.html">glossary</a>. See also our <a +href="biblio.html">bibliography</a> and cryptography <a +href="web.html#crypto.link">web links</a>.</p> + +<h3><a name="block.cipher">Block ciphers</a></h3> + +<p>The <a href="glossary.html#encryption">encryption</a> in the <a +href="glossary.html#ESP">ESP</a> encapsulation protocol is done with a <a +href="glossary.html#block">block cipher</a>.</p> + +<p>We do not implement <a href="glossary.html#DES">single DES</a>. It is <a +href="politics.html#desnotsecure">insecure</a>. Our default, and currently +only, block cipher is <a href="glossary.html#3DES">triple DES</a>.</p> + +<p>The <a href="glossary.html#rijndael">Rijndael</a> block cipher has won the +<a href="glossary.html#AES">AES</a> competition to choose a relacement for +DES. It will almost certainly be added to FreeS/WAN and to other IPsec +implementations. <a href="web.html#patch">Patches</a> are already +available.</p> + +<h3><a name="hash.ipsec">Hash functions</a></h3> + +<h4><a name="hmac.ipsec">The HMAC construct</a></h4> + +<p>IPsec packet authentication is done with the <a +href="glossary.html#HMAC">HMAC</a> construct. This is not just a hash of the +packet data, but a more complex operation which uses both a hashing algorithm +and a key. It therefore does more than a simple hash would. A simple hash +would only tell you that the packet data was not changed in transit, or that +whoever changed it also regenerated the hash. An HMAC also tells you that the +sender knew the HMAC key.</p> + +<p>For IPsec HMAC, the output of the hash algorithm is truncated to 96 bits. +This saves some space in the packets. More important, it prevents an attacker +from seeing all the hash output bits and perhaps creating some sort of attack +based on that knowledge.</p> + +<h4>Choice of hash algorithm</h4> + +<p>The IPsec RFCs require two hash algorithms -- <a +href="glossary.html#MD5">MD5</a> and <a href="glossary.html#SHA">SHA-1</a> -- +both of which FreeS/WAN implements.</p> + +<p>Various other algorithms -- such as RIPEMD and Tiger -- are listed in the +RFCs as optional. None of these are in the FreeS/WAN distribution, or are +likely to be added, although user <a href="web.html#patch">patches</a> exist +for several of them.</p> + +<p>Additional hash algorithms -- <a href="glossary.html#SHA-256">SHA-256, +SHA-384 and SHA-512</a> -- may be required to give hash strength matching the +strength of <a href="glossary.html#AES">AES</a>. These are likely to be added +to FreeS/WAN along with AES.</p> + +<h3><a name="DH.keying">Diffie-Hellman key agreement</a></h3> + +<p>The <a href="glossary.html#DH">Diffie-Hellman</a> key agreement protocol +allows two parties (A and B or <a href="glossary.html#alicebob">Alice and +Bob</a>) to agree on a key in such a way that an eavesdropper who intercepts +the entire conversation cannot learn the key.</p> + +<p>The protocol is based on the <a href="glossary.html#dlog">discrete +logarithm</a> problem and is therefore thought to be secure. Mathematicians +have been working on that problem for years and seem no closer to a solution, +though there is no proof that an efficient solution is impossible.</p> + +<h3><a name="RSA.auth">RSA authentication</a></h3> + +<p>The <a href="glossary.html#RSA">RSA</a> algorithm (named for its inventors +-- Rivest, Shamir and Adleman) is a very widely used <a +href="glossary.html#">public key</a> cryptographic technique. It is used in +IPsec as one method of authenticating gateways for Diffie-Hellman key +negotiation.</p> + +<h2><a name="structure">Structure of IPsec</a></h2> + +<p>There are three protocols used in an IPsec implementation:</p> +<dl> + <dt>ESP, Encapsulating Security Payload</dt> + <dd>Encrypts and/or authenticates data</dd> + <dt>AH, Authentication Header</dt> + <dd>Provides a packet authentication service</dd> + <dt>IKE, Internet Key Exchange</dt> + <dd>Negotiates connection parameters, including keys, for the other + two</dd> +</dl> + +<p>The term "IPsec" is slightly ambiguous. In some contexts, it includes all +three of the above but in other contexts it refers only to AH and ESP.</p> + +<h3><a name="IKE.ipsec">IKE (Internet Key Exchange)</a></h3> + +<p>The IKE protocol sets up IPsec (ESP or AH) connections after negotiating +appropriate parameters (algorithms to be used, keys, connection lifetimes) +for them. This is done by exchanging packets on UDP port 500 between the two +gateways.</p> + +<p>IKE (RFC 2409) was the outcome of a long, complex process in which quite a +number of protocols were proposed and debated. Oversimplifying mildly, IKE +combines:</p> +<dl> + <dt>ISAKMP (RFC 2408)</dt> + <dd>The <strong>I</strong>nternet <strong>S</strong>ecurity + <strong>A</strong>ssociation and <strong>K</strong>ey + <strong>M</strong>anagement <strong>P</strong>rotocol manages + negotiation of connections and defines <a + href="glossary.html#SA">SA</a>s (Security Associations) as a means of + describing connection properties.</dd> + <dt>IPsec DOI for ISAKMP (RFC 2407)</dt> + <dd>A <strong>D</strong>omain <strong>O</strong>f + <strong>I</strong>nterpretation fills in the details necessary to turn + the rather abstract ISAKMP protocol into a more tightly specified + protocol, so it becomes applicable in a particular domain.</dd> + <dt>Oakley key determination protocol (RFC 2412)</dt> + <dd>Oakley creates keys using the <a + href="glossary.html#DH">Diffie-Hellman</a> key agreement protocol.</dd> +</dl> + +<p>For all the details, you would need to read the four <a +href="rfc.html">RFCs</a> just mentioned (over 200 pages) and a number of +others. We give a summary below, but it is far from complete.</p> + +<h4><a name="phases">Phases of IKE</a></h4> + +<p>IKE negotiations have two phases.</p> +<dl> + <dt>Phase one</dt> + <dd>The two gateways negotiate and set up a two-way ISAKMP SA which they + can then use to handle phase two negotiations. One such SA between a + pair of gateways can handle negotiations for multiple tunnels.</dd> + <dt>Phase two</dt> + <dd>Using the ISAKMP SA, the gateways negotiate IPsec (ESP and/or AH) SAs + as required. IPsec SAs are unidirectional (a different key is used in + each direction) and are always negotiated in pairs to handle two-way + traffic. There may be more than one pair defined between two + gateways.</dd> +</dl> + +<p>Both of these phases use the UDP protocol and port 500 for their +negotiations.</p> + +<p>After both IKE phases are complete, you have IPsec SAs to carry your +encrypted data. These use the ESP or AH protocols. These protocols do not +have ports. Ports apply only to UDP or TCP.</p> + +<p>The IKE protocol is designed to be extremely flexible. Among the things +that can be negotiated (separately for each SA) are:</p> +<ul> + <li>SA lifetime before rekeying</li> + <li>encryption algorithm used. We currently support only <a + href="glossary.html#3DES">triple DES</a>. Single DES is <a + href="politics.html#desnotsecure">insecure</a>. The RFCs say you MUST do + DES, SHOULD do 3DES and MAY do various others. We do not do any of the + others.</li> + <li>authentication algorithms. We support <a + href="glossary.html#MD5">MD5</a> and <a href="glossary.html#SHA">SHA</a>. + These are the two the RFCs require.</li> + <li>choice of group for <a href="glossary.html#DH">Diffie-Hellman</a> key + agreement. We currently support Groups 2 and 5 (which are defined modulo + primes of various lengths) and do not support Group 1 (defined modulo a + shorter prime, and therefore cryptographically weak) or groups 3 and 4 + (defined using elliptic curves). The RFCs require only Group 1.</li> +</ul> + +<p>The protocol also allows implementations to add their own encryption +algorithms, authentication algorithms or Diffie-Hellman groups. We do not +support any such extensions, but there are some <a +href="web.html#patch">patches</a> that do.</p> + +<p>There are a number of complications:</p> +<ul> + <li>The gateways must be able to authenticate each other's identities + before they can create a secure connection. This host authentication is + part of phase one negotiations, and is a required prerequisite for packet + authentication used later. Host authentication can be done in a variety + of ways. Those supported by FreeS/WAN are discussed in our <a + href="adv_config.html#auto-auth">advanced configuration</a> document.</li> + <li>Phase one can be done in two ways. + <ul> + <li>Main Mode is required by the RFCs and supported in FreeS/WAN. It + uses a 6-packet exzchange.</li> + <li>Aggressive Mode is somewhat faster (only 3 packets) but reveals + more to an eavesdropper. This is optional in the RFCs, not currently + supported by FreeS/WAN, and not likely to be.</li> + </ul> + </li> + <li>A new group exchange may take place after phase one but before phase + two, defining an additional group for use in the <a + href="glossary.html#DH">Diffie-Hellman</a> key agreement part of phase + two. FreeS/WAN does not currently support this.</li> + <li>Phase two always uses Quick Mode, but there are two variants of that: + <ul> + <li>One variant provides <a href="glossary.html#PFS">Perfect Forward + Secrecy (PFS)</a>. An attacker that obtains your long-term host + authentication key does not immediately get any of your short-term + packet encryption of packet authentication keys. He must conduct + another successful attack each time you rekey to get the short-term + keys. Having some short-term keys does not help him learn others. In + particular, breaking your system today does not let him read messages + he archived yestarday, assuming you've changed short-term keys in the + meanwhile. We enable PFS as the default.</li> + <li>The other variant disables PFS and is therefore slightly faster. We + do not recommend this since it is less secure, but FreeS/WAN does + support it. You can enable it with a <var>pfs=no</var> statement in + <a href="manpage.d/ipsec.conf.5.html">ipsec.conf(5)</a>.</li> + <li>The protocol provides no way to negotiate which variant will be + used. If one gateway is set for PFS and the other is not, the + negotiation fails. This has proved a fairly common source of + interoperation problems.</li> + </ul> + </li> + <li>Several types of notification message may be sent by either side during + either phase, or later. FreeS/WAN does not currently support these, but + they are a likely addition in future releases.</li> + <li>There is a commit flag which may optionally be set on some messages. + The <a href="http://www.lounge.org/ike_doi_errata.html">errata</a> page + for the RFCs includes two changes related to this, one to clarify the + description of its use and one to block a <a + href="glossary.html#DOS">denial of service</a> attack which uses it. We + currently do not implement this feature.</li> +</ul> + +<p>These complications can of course lead to problems, particularly when two +different implementations attempt to interoperate. For example, we have seen +problems such as:</p> +<ul> + <li>Some implementations (often products crippled by <a + href="politics.html#exlaw">export laws</a>) have the insecure DES + algorithm as their only supported encryption method. Other parts of our + documentation discuss the <a + href="politics.html#desnotsecure">reasons we do not implement single + DES</a>, and <a href="interop.html#noDES">how to cope with crippled + products</a>.</li> + <li>Windows 2000 IPsec tries to negotiate using Aggressive Mode, which we + don't support. Later on, it uses the commit bit, which we also don't + support.</li> + <li>Various implementations disable PFS by default, and therefore will not + talk to FreeS/WAN until you either turn on PFS on their end or turn it + off in FreeS/WAN with a <var>pfs=no</var> entry in the connection + description.</li> + <li>FreeS/WAN's interaction with PGPnet is complicated by their use of + notification messages we do not yet support.</li> +</ul> + +<p>Despite this, we do interoperate successfully with many implementations, +including both Windows 2000 and PGPnet. Details are in our <a +href="interop.html">interoperability</a> document.</p> + +<h4><a name="sequence">Sequence of messages in IKE</a></h4> + +<p>Each phase (see <a href="#phases">previous section</a>)of IKE involves a +series of messages. In Pluto error messages, these are abbreviated using:</p> +<dl> + <dt>M</dt> + <dd><strong>M</strong>ain mode, settting up the keying channel (ISAKMP + SA)</dd> + <dt>Q</dt> + <dd><strong>Q</strong>uick mode, setting up the data channel (IPsec + SA)</dd> + <dt>I</dt> + <dd><strong>I</strong>nitiator, the machine that starts the + negotiation</dd> + <dt>R</dt> + <dd><strong>R</strong>esponder</dd> +</dl> + +<p>For example, the six messages of a main mode negotiation, in sequence, are +labelled:</p> +<pre> MI1 ----------> + <---------- MR1 + MI2 ----------> + <---------- MR2 + MI3 ----------> + <---------- MR3</pre> + +<h4><a name="struct.exchange">Structure of IKE messages</a></h4> + +<p>Here is our Pluto developer explaining some of this on the mailing +list:</p> +<pre>When one IKE system (for example, Pluto) is negotiating with another +to create an SA, the Initiator proposes a bunch of choices and the +Responder replies with one that it has selected. + +The structure of the choices is fairly complicated. An SA payload +contains a list of lists of "Proposals". The outer list is a set of +choices: the selection must be from one element of this list. + +Each of these elements is a list of Proposals. A selection must be +made from each of the elements of the inner list. In other words, +*all* of them apply (that is how, for example, both AH and ESP can +apply at once). + +Within each of these Proposals is a list of Transforms. For each +Proposal selected, one Transform must be selected (in other words, +each Proposal provides a choice of Transforms). + +Each Transform is made up of a list of Attributes describing, well, +attributes. Such as lifetime of the SA. Such as algorithm to be +used. All the Attributes apply to a Transform. + +You will have noticed a pattern here: layers alternate between being +disjunctions ("or") and conjunctions ("and"). + +For Phase 1 / Main Mode (negotiating an ISAKMP SA), this structure is +cut back. There must be exactly one Proposal. So this degenerates to +a list of Transforms, one of which must be chosen.</pre> + +<h3><a name="services">IPsec Services, AH and ESP</a></h3> + +<p>IPsec offers two services, <a +href="glossary.html#authentication">authentication</a> and <a +href="glossary.html#encryption">encryption</a>. These can be used separately +but are often used together.</p> +<dl> + <dt>Authentication</dt> + <dd>Packet-level authentication allows you to be confident that a packet + came from a particular machine and that its contents were not altered + en route to you. No attempt is made to conceal or protect the contents, + only to assure their integrity. Packet authentication can be provided + separately using an <a href="glossary.html#AH">Authentication + Header</a>, described just below, or it can be included as part of the + <a href="glossary.html#ESP">ESP</a> (Encapsulated Security Payload) + service, described in the following section. That service offers + encryption as well as authentication. In either case, the <a + href="glossary.html#HMAC">HMAC</a> construct is used as the + authentication mechanism. + <p>There is a separate authentication operation at the IKE level, in + which each gateway authenticates the other. This can be done in a + variety of ways.</p> + </dd> + <dt>Encryption</dt> + <dd>Encryption allows you to conceal the contents of a message from + eavesdroppers. + <p>In IPsec this is done using a <a href="glossary.html#block">block + cipher</a> (normally <a href="glossary.html#3DES">Triple DES</a> for + Linux). In the most used setup, keys are automatically negotiated, and + periodically re-negotiated, using the <a + href="glossary.html#IKE">IKE</a> (Internet Key Exchange) protocol. In + Linux FreeS/WAN this is handled by the Pluto Daemon.</p> + <p>The IPsec protocol offering encryption is <a + href="glossary.html#ESP">ESP</a>, Encapsulated Security Payload. It can + also include a packet authentication service.</p> + </dd> +</dl> + +<p>Note that <strong>encryption should always be used with some packet +authentication service</strong>. Unauthenticated encryption is vulnerable to +<a href="glossary.html#middle">man-in-the-middle attacks</a>. Also note that +encryption does not prevent <a href="glossary.html#traffic">traffic +analysis</a>.</p> + +<h3><a name="AH.ipsec">The Authentication Header (AH)</a></h3> + +<p>Packet authentication can be provided separately from encryption by adding +an authentication header (AH) after the IP header but before the other +headers on the packet. This is the subject of this section. Details are in +RFC 2402.</p> + +<p>Each of the several headers on a packet header contains a "next protocol" +field telling the system what header to look for next. IP headers generally +have either TCP or UDP in this field. When IPsec authentication is used, the +packet IP header has AH in this field, saying that an Authentication Header +comes next. The AH header then has the next header type -- usually TCP, UDP +or encapsulated IP.</p> + +<p>IPsec packet authentication can be added in transport mode, as a +modification of standard IP transport. This is shown in this diagram from the +RFC:</p> +<pre> BEFORE APPLYING AH + ---------------------------- + IPv4 |orig IP hdr | | | + |(any options)| TCP | Data | + ---------------------------- + + AFTER APPLYING AH + --------------------------------- + IPv4 |orig IP hdr | | | | + |(any options)| AH | TCP | Data | + --------------------------------- + || + except for mutable fields</pre> + +<p>Athentication can also be used in tunnel mode, encapsulating the +underlying IP packet beneath AH and an additional IP header.</p> +<pre> || +IPv4 | new IP hdr* | | orig IP hdr* | | | + |(any options)| AH | (any options) |TCP | Data | + ------------------------------------------------ + || + | in the new IP hdr |</pre> + +<p>This would normally be used in a gateway-to-gateway tunnel. The receiving +gateway then strips the outer IP header and the AH header and forwards the +inner IP packet.</p> + +<p>The mutable fields referred to are things like the time-to-live field in +the IP header. These cannot be included in authentication calculations +because they change as the packet travels.</p> + +<h4><a name="keyed">Keyed MD5 and Keyed SHA</a></h4> + +<p>The actual authentication data in the header is typically 96 bits and +depends both on a secret shared between sender and receiver and on every byte +of the data being authenticated. The technique used is <a +href="glossary.html#HMAC">HMAC</a>, defined in RFC 2104.</p> + +<p>The algorithms involved are the <a href="glossary.html#MD5">MD5</a> +Message Digest Algorithm or <a href="glossary.html#SHA">SHA</a>, the Secure +Hash Algorithm. For details on their use in this application, see RFCs 2403 +and 2404 respectively.</p> + +<p>For descriptions of the algorithms themselves, see RFC 1321 for MD5 and <a +href="glossary.html#FIPS">FIPS</a> (Federal Information Processing Standard) +number 186 from <a href="glossary.html#NIST">NIST</a>, the US National +Institute of Standards and Technology for SHA. <a +href="biblio.html#schneier"><cite>Applied Cryptography</cite></a> covers both +in some detail, MD5 starting on page 436 and SHA on 442.</p> + +<p>These algorithms are intended to make it nearly impossible for anyone to +alter the authenticated data in transit. The sender calculates a digest or +hash value from that data and includes the result in the authentication +header. The recipient does the same calculation and compares results. For +unchanged data, the results will be identical. The hash algorithms are +designed to make it extremely difficult to change the data in any way and +still get the correct hash.</p> + +<p>Since the shared secret key is also used in both calculations, an +interceptor cannot simply alter the authenticated data and change the hash +value to match. Without the key, he or she (or even the dreaded They) cannot +produce a usable hash.</p> + +<h4><a name="sequence">Sequence numbers</a></h4> + +<p>The authentication header includes a sequence number field which the +sender is required to increment for each packet. The receiver can ignore it +or use it to check that packets are indeed arriving in the expected +sequence.</p> + +<p>This provides partial protection against <a +href="glossary.html#replay">replay attacks</a> in which an attacker resends +intercepted packets in an effort to confuse or subvert the receiver. Complete +protection is not possible since it is necessary to handle legitmate packets +which are lost, duplicated, or delivered out of order, but use of sequence +numbers makes the attack much more difficult.</p> + +<p>The RFCs require that sequence numbers never cycle, that a new key always +be negotiated before the sequence number reaches 2^32-1. This protects both +against replays attacks using packets from a previous cyclce and against <a +href="glossary.html#birthday">birthday attacks</a> on the the packet +authentication algorithm.</p> + +<p>In Linux FreeS/WAN, the sequence number is ignored for manually keyed +connections and checked for automatically keyed ones. In manual mode, there +is no way to negotiate a new key, or to recover from a sequence number +problem, so we don't use sequence numbers.</p> + +<h3><a name="ESP.ipsec">Encapsulated Security Payload (ESP)</a></h3> + +<p>The ESP protocol is defined in RFC 2406. It provides one or both of +encryption and packet authentication. It may be used with or without AH +packet authentication.</p> + +<p>Note that <strong>some form of packet authentication should +<em>always</em> be used whenever data is encrypted</strong>. Without +authentication, the encryption is vulnerable to active attacks which may +allow an enemy to break the encryption. ESP should <strong>always</strong> +either include its own authentication or be used with AH authentication.</p> + +<p>The RFCs require support for only two mandatory encryption algorithms -- +<a href="glossary.html#DES">DES</a>, and null encryption -- and for two +authentication methods -- keyed MD5 and keyed SHA. Implementers may choose to +support additional algorithms in either category.</p> + +<p>The authentication algorithms are the same ones used in the IPsec <a +href="#AH">authentication header</a>.</p> + +<p>We do not implement single DES since <a +href="politics.html#desnotsecure">DES is insecure</a>. Instead we provide <a +href="glossary.html#3DES">triple DES or 3DES</a>. This is currently the only +encryption algorithm supported.</p> + +<p>We do not implement null encryption since it is obviously insecure.</p> + +<h2><a name="modes">IPsec modes</a></h2> + +<p>IPsec can connect in two modes. Transport mode is a host-to-host +connection involving only two machines. In tunnel mode, the IPsec machines +act as gateways and trafiic for any number of client machines may be +carried.</p> + +<h3><a name="tunnel.ipsec">Tunnel mode</a></h3> + +<p>Security gateways are required to support tunnel mode connections. In this +mode the gateways provide tunnels for use by client machines behind the +gateways. The client machines need not do any IPsec processing; all they have +to do is route things to gateways.</p> + +<h3><a name="transport.ipsec">Transport mode</a></h3> + +<p>Host machines (as opposed to security gateways) with IPsec implementations +must also support transport mode. In this mode, the host does its own IPsec +processing and routes some packets via IPsec.</p> + +<h2><a name="parts">FreeS/WAN parts</a></h2> + +<h3><a name="KLIPS.ipsec">KLIPS: Kernel IPsec Support</a></h3> + +<p>KLIPS is <strong>K</strong>erne<strong>L</strong> <strong>IP</strong>SEC +<strong>S</strong>upport, the modifications necessary to support IPsec within +the Linux kernel. KILPS does all the actual IPsec packet-handling, +including</p> +<ul> + <li>encryption</li> + <li>packet authentication calculations</li> + <li>creation of ESP and AH headers for outgoing packets</li> + <li>interpretation of those headers on incoming packets</li> +</ul> + +<p>KLIPS also checks all non-IPsec packets to ensure they are not bypassing +IPsec security policies.</p> + +<h3><a name="Pluto.ipsec">The Pluto daemon</a></h3> + +<p><a href="manpage.d/ipsec_pluto.8.html">Pluto(8)</a> is a daemon which +implements the IKE protocol. It</p> +<ul> + <li>handles all the Phase one ISAKMP SAs</li> + <li>performs host authentication and negotiates with other gateways</li> + <li>creates IPsec SAs and passes the data required to run them to KLIPS</li> + <li>adjust routing and firewall setup to meet IPsec requirements. See our + <a href="firewall.html">IPsec and firewalling</a> document for + details.</li> +</ul> + +<p>Pluto is controlled mainly by the <a +href="manpage.d/ipsec.conf.5.html">ipsec.conf(5)</a> configuration file.</p> + +<h3><a name="command">The ipsec(8) command</a></h3> + +<p>The <a href="manpage.d/ipsec.8.html">ipsec(8)</a> command is a front end +shellscript that allows control over IPsec activity.</p> + +<h3><a name="ipsec.conf">Linux FreeS/WAN configuration file</a></h3> + +<p>The configuration file for Linux FreeS/WAN is</p> +<pre> /etc/ipsec.conf</pre> + +<p>For details see the <a +href="manpage.d/ipsec.conf.5.html">ipsec.conf(5)</a> manual page .</p> + +<h2><a name="key">Key management</a></h2> + +<p>There are several ways IPsec can manage keys. Not all are implemented in +Linux FreeS/WAN.</p> + +<h3><a name="current">Currently Implemented Methods</a></h3> + +<h4><a name="manual">Manual keying</a></h4> + +<p>IPsec allows keys to be manually set. In Linux FreeS/WAN, such keys are +stored with the connection definitions in /etc/ipsec.conf.</p> + +<p><a href="glossary.html#manual">Manual keying</a> is useful for debugging +since it allows you to test the <a href="glossary.html#KLIPS">KLIPS</a> +kernel IPsec code without the <a href="glossary.html#Pluto">Pluto</a> daemon +doing key negotiation.</p> + +<p>In general, however, automatic keying is preferred because it is more +secure.</p> + +<h4><a name="auto">Automatic keying</a></h4> + +<p>In automatic keying, the <a href="glossary.html#Pluto">Pluto</a> daemon +negotiates keys using the <a href="glossary.html#IKE">IKE</a> Internet Key +Exchange protocol. Connections are automatically re-keyed periodically.</p> + +<p>This is considerably more secure than manual keying. In either case an +attacker who acquires a key can read every message encrypted with that key, +but automatic keys can be changed every few hours or even every few minutes +without breaking the connection or requiring intervention by the system +administrators. Manual keys can only be changed manually; you need to shut +down the connection and have the two admins make changes. Moreover, they have +to communicate the new keys securely, perhaps with <a +href="glossary.html#PGP">PGP</a> or <a href="glossary.html#SSH">SSH</a>. This +may be possible in some cases, but as a general solution it is expensive, +bothersome and unreliable. Far better to let <a +href="glossary.html#Pluto">Pluto</a> handle these chores; no doubt the +administrators have enough to do.</p> + +<p>Also, automatic keying is inherently more secure against an attacker who +manages to subvert your gateway system. If manual keying is in use and an +adversary acquires root privilege on your gateway, he reads your keys from +/etc/ipsec.conf and then reads all messages encrypted with those keys.</p> + +<p>If automatic keying is used, an adversary with the same privileges can +read /etc/ipsec.secrets, but this does not contain any keys, only the secrets +used to authenticate key exchanges. Having an adversary able to authenticate +your key exchanges need not worry you overmuch. Just having the secrets does +not give him any keys. You are still secure against <a +href="glossary.html#passive">passive</a> attacks. This property of automatic +keying is called <a href="glossary.html#PFS">perfect forward secrecy</a>, +abbreviated PFS.</p> + +<p>Unfortunately, having the secrets does allow an <a +href="glossary.html#active">active attack</a>, specifically a <a +href="glossary.html#middle">man-in-the-middle</a> attack. Losing these +secrets to an attacker may not be quite as disastrous as losing the actual +keys, but it is <em>still a serious security breach</em>. These secrets +should be guarded as carefully as keys.</p> + +<h3><a name="notyet">Methods not yet implemented</a></h3> + +<h4><a name="noauth">Unauthenticated key exchange</a></h4> + +<p>It would be possible to exchange keys without authenticating the players. +This would support <a href="glossary.html#carpediem">opportunistic +encryption</a> -- allowing any two systems to encrypt their communications +without requiring a shared PKI or a previously negotiated secret -- and would +be secure against <a href="glossary.html#passive">passive attacks</a>. It +would, however, be highly vulnerable to active <a +href="glossary.html#middle">man-in-the-middle</a> attacks. RFC 2408 therefore +specifies that all <a href="glossary.html#ISAKMP">ISAKMP</a> key management +interactions <em>must</em> be authenticated.</p> + +<p>There is room for debate here. Should we provide immediate security +against <a href="glossary.html#passive">passive attacks</a> and encourage +widespread use of encryption, at the expense of risking the more difficult <a +href="glossary.html#active">active attacks</a>? Or should we wait until we +can implement a solution that can both be widespread and offer security +against active attacks?</p> + +<p>So far, we have chosen the second course, complying with the RFCs and +waiting for secure DNS (see <a href="glossary.html#DNS">below</a>) so that we +can do <a href="glossary.html#carpediem">opportunistic encryption</a> +right.</p> + +<h4><a name="DNS">Key exchange using DNS</a></h4> + +<p>The IPsec RFCs allow key exchange based on authentication services +provided by <a href="glossary.html#SDNS">Secure DNS</a>. Once Secure DNS +service becomes widely available, we expect to make this the <em>primary key +management method for Linux FreeS/WAN</em>. It is the best way we know of to +support <a href="glossary.html#carpediem">opportunistic encryption</a>, +allowing two systems without a common PKI or previous negotiation to secure +their communication.</p> + +<p>We currently have code to acquire RSA keys from DNS but do not yet have +code to validate Secure DNS signatures.</p> + +<h4><a name="PKI">Key exchange using a PKI</a></h4> + +<p>The IPsec RFCs allow key exchange based on authentication services +provided by a <a href="glossary.html#PKI">PKI</a> or Public Key +Infrastructure. With many vendors selling such products and many large +organisations building these infrastructures, this will clearly be an +important application of IPsec and one Linux FreeS/WAN will eventually +support.</p> + +<p>On the other hand, this is not as high a priority for Linux FreeS/WAN as +solutions based on <a href="glossary.html#SDNS">secure DNS</a>. We do not +expect any PKI to become as universal as DNS.</p> + +<p>Some <a href="web.html#patch">patches</a> to handle authentication with +X.509 certificates, which most PKIs use, are available.</p> + +<h4><a name="photuris">Photuris</a></h4> + +<p><a href="glossary.html#photuris">Photuris</a> is another key management +protocol, an alternative to IKE and ISAKMP, described in RFCs 2522 and 2523 +which are labelled "experimental". Adding Photuris support to Linux FreeS/WAN +might be a good project for a volunteer. The likely starting point would be +the OpenBSD photurisd code.</p> + +<h4><a name="skip">SKIP</a></h4> + +<p><a href="glossary.html#SKIP">SKIP</a> is yet another key management +protocol, developed by Sun. At one point it was fairly widely used, but it +now seems moribund, displaced by IKE. Sun now (as of Solaris 8.0) ship an +IPsec implementation using IKE. We have no plans to implement SKIP. If a user +were to implement it, we would almost certainly not want to add the code to +our distribution.</p> +</body> +</html> |